Image sensor pixels with conductive bias grids

ABSTRACT

An image sensor with an array of pixels is provided. In order to achieve high image quality, it may be desirable to improve well capacity of individual pixels within the array by forming deep photodiodes in a thick substrate. When forming the array of pixels, conductive contacts may be formed in a back surface of the substrate opposing ground contacts located on a front side of the substrate. A conductive grid layer may be formed over the conductive contacts. A color filter layer may be formed over the conductive grid layer that may include a barrier grid in which color filter material is deposited. The conductive grid layer and conductive contacts may be biased to a voltage to improve the strength of electric fields in the substrate. Conductive contacts will thereby improve charge collection and electrical isolation and prevent electrical crosstalk and blooming.

BACKGROUND

This relates generally to image sensors, and more specifically toback-side-illuminated (BSI) image sensors with conductive bias grids toenhance charge collection and improve photodiode isolation.

Image sensors are commonly used in electronic devices such as cellulartelephones, cameras, and computers to capture images. Conventional imagesensors are fabricated on a semiconductor substrate using complementarymetal-oxide-semiconductor (CMOS) technology or charge-coupled device(CCD) technology. The image sensors may include an array of image sensorpixels each of which includes a photodiode and other operationalcircuitry such as transistors formed in the substrate.

Image sensors often include a photodiode having a pinning-voltage whichis a design parameter set by the doping levels of the photodiode. Duringnormal operation, a photodiode node is first reset to thepinning-voltage using transistor circuitry. Photons are then allowed toenter the photodiode region for a given amount of time. The photons areconverted to charge carriers inside the photodiode, and these chargecarriers reduce the reset pinning-voltage. In this process, the maximumtotal charge stored, Q_(MAX), is commonly referred to as the saturationfull well (SFW) and depends on the well capacity of the photodiode. Theactual charge stored, Q, is less than or equal to Q_(MAX) based on theintensity and integration time of photons. When it is time to read outthe stored signal, the stored charge Q at the photodiode node istransferred to a floating diffusion node through additional transistorcircuitry. If care is not taken to maximize the amount of charge Q thancan be transferred from the photodiode to the floating diffusion node,charge spill back can degrade image quality. Maximum charge stored,Q_(MAX), determines the highest signal level detected in the photodiodearray. High Q_(MAX) improves dynamic range of an image sensor, lowersthe noise floor, and can be used to improve saturation artifacts.

A deep photodiode (DPD) process can be used to increase the quantumefficiency and Q_(MAX) characteristics of a pixel, which have anincreasing impact on pixel performance as pixel area decreases. Inconventional image sensors, the electric field strength of groundelectrodes in a pixel is weakened as the substrate depth of the pixelincreases. The DPD process requires a thicker substrate than traditionalpixel depth processes and also requires the addition of an additionalresistive path, which further decreases the electric field magnitude ofthe ground contacts as substrate depth increases. As a consequence ofthe decreased electric field strength, the p-n junction close to theback-side interface of the pixel is not in strong reverse bias, whichleads to low charge collection efficiency and weak electrical isolation.The lowered charge collection efficiency occurs due to an increasedprobability for charge carriers to recombine before reaching the groundcontacts. Electrical crosstalk and blooming may occur as a result ofweak electrical isolation.

It would therefore be desirable to be able to provide BSI image sensorswith enhanced charge collection and improved photodiode isolation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative electronic device in accordancewith an embodiment.

FIG. 2 is a cross-sectional side view of a conventionalback-side-illuminated (BSI) image pixel array.

FIG. 3 is cross-sectional side view of an illustrative BSI image pixelarray showing conductive substrate contacts, a bias grid layer,dielectric color filter barriers, and a wire-bond edge interface inaccordance with an embodiment.

FIG. 4 is a cross-sectional side view of an illustrative BSI image pixelarray showing conductive substrate contacts, a bias grid layer, metalcolor filter barriers, and a through-hole via edge interface inaccordance with an embodiment.

FIG. 5 is a top-down view of an illustrative image pixel array showingcontacts located at each intersection of a bias grid under a Bayer colorfilter array in accordance with an embodiment.

FIG. 6 is a top-down view of an illustrative image pixel array showingshared contacts located at alternating intersections of a bias gridunder a Bayer color filter array in accordance with an embodiment.

FIG. 7 is a flow chart of illustrative steps involved in fabricating aBSI image pixel array having conductive substrate contacts, a bias gridlayer, and a grid of dielectric color filter barriers in accordance withan embodiment.

FIG. 8 is a flow chart of illustrative steps involved in fabricating aBSI image pixel array having conductive substrate contacts and a biasgrid of metal color filter barriers in accordance with an embodiment.

FIG. 9 is a block diagram of a processor system employing at least someof the embodiments of the image pixel array in FIGS. 3-6 in accordancewith an embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention relate to image sensors, and morespecifically to back-side-illuminated (BSI) image sensors withconductive bias grids to enhance charge collection and improvephotodiode isolation. It will be recognized by one skilled in the art,that the present exemplary embodiments may be practiced without some orall of these specific details. In other instances, well-known operationshave not been described in detail in order not to unnecessarily obscurethe present embodiments.

FIG. 1 is a diagram of an illustrative electronic device that uses animage sensor to capture images. Imaging system 10 of FIG. 1 may be aportable imaging system such as a camera, a cellular telephone, a videocamera, or other imaging device that captures digital image data. Cameramodule 12 may be used to convert incoming light into digital image data.Camera module 12 may include a lens 14 and a corresponding image sensor16. Lens 14 and image sensor 16 may be mounted in a common package andmay provide image data to processing circuitry 18. In some embodimentslens 14 may be part of an array of lenses and image sensor 16 may bepart of an image sensor array.

Processing circuitry 18 may include one or more integrated circuits(e.g., image processing circuits, microprocessors, storage devices suchas random-access memory and non-volatile memory, etc.) and may beimplemented using components that are separate from camera module 12and/or that form part of camera module 12 (e.g., circuits that form partof an integrated circuit that includes image sensor 16 or an integratedcircuit within module 12 that is associated with image sensor 16). Imagedata that has been captured and processed by camera module 12 may, ifdesired, be further processed and stored using processing circuitry 18.Processed image data may, if desired, be provided to external equipment(e.g., a computer or other device) using wired and/or wirelesscommunications paths coupled to processing circuitry 18.

Image sensor 16 may be configured to receive light of a given color byproviding the image sensor with a color filter. The color filters thatare used for image sensor pixel arrays in the image sensor may, forexample, be red filters, blue filters, and green filters. Each filtermay form a color filter layer that covers the image sensor pixel arrayof the image sensor. Other filters such as white color filters,dual-band IR cutoff filters (e.g., filters that allow visible light anda range of infrared light emitted by LED lights), etc. may also be used.

FIG. 2 is a cross-sectional side view of a conventionalback-side-illuminated (BSI) image pixel array. As shown in FIG. 2, imagepixel array 200 includes a color filter array 210 formed on a backsurface of a substrate 220. An array of microlenses 204 is formed overthe color filter array 210. Color filter array 210 includes color filtermaterial 212 deposited in cavities of dielectric color filter barriergrid 214. Substrate 220 includes active photodiode regions 224 having aheight D1 and being formed in lightly doped semiconductor region 222.Transfer gates 206 and ground contacts 208 are formed on the frontsurface of substrate 220.

During operation, photons 202 are permitted to enter through microlensarray 204 and color filter array 210 for a pre-defined amount of time.It is desirable for a majority of the photons 202 that enter image pixelarray 200 to generate electron-hole pairs inside active photodioderegions 224. Active photodiode regions 224 may store an electron chargeQ during this time period. The magnitude of charge that is stored inactive photodiode regions 224 is limited by the saturated-full-wellcapacity of active photodiode regions 224. The charge Q may then betransferred from active photodiode regions 224 to floating diffusionnodes with transfer gates 206.

The embodiment shown in FIG. 2 has reduced utility when pixel area issmall. Because of the shallow depth D1 of substrate 220, quantumefficiency and full well capacity of image pixel array 200 are limited.If substrate depth were to be increased, the magnitude of electricfields in substrate 220 associated with ground contacts 208 would beweakened in correlation with the increase in depth. As a consequence ofthe weakened electric fields, portions of photodiode regions 224 closestto the back surface of substrate 220 would not be in strong reversebias, which would lead to low charge collection efficiency, electricalcrosstalk, and blooming. It would therefore be desirable to increasesubstrate depth D1 in image pixel array 200 without sacrificing electricfield strength in substrate 220.

FIG. 3 is a cross-sectional side view of an illustrative BSI image pixelarray showing conductive substrate contacts, a bias grid layer,dielectric color filter barriers, and a wire-bond edge interface inaccordance with an embodiment. As shown in FIG. 3, image pixel array 300may receive photons 302 and may include an image pixel array edge 330and a color filter 310 formed over a conductive bias grid layer 328 thatmay be formed on a back surface of a substrate 320. Substrate 320 mayconsist of semiconductor material (e.g., silicon, silicon carbide,gallium nitride, gallium arsenide, etc.). Conductive bias grid layer 328may be formed from metal material (e.g., aluminum, tungsten, copper,etc.), conductive oxides (e.g., indium tin oxide, bismuth titanate), ordoped semiconductor material. An array of microlenses 304 may be formedover color filter array 310. Ground contacts 308 and transfer gates 306may be formed on a front surface of semiconductor 320. Ground contacts308 may be formed from conductive material (e.g., aluminum, tungsten,copper) and/or may be formed over a doped portion of substrate 320.

Color filter array 310 may include color filter material 312 that isdeposited in cavities of color filter barrier grid 314. In theillustrative example of FIG. 3, color filter barrier grid 314 is formedfrom dielectric material. Each cell in color filter barrier grid 314 maydefine one or more individual pixels in image pixel array 300. Colorfilter barrier grid 314 may be formed directly over conductive bias gridlayer 328 and may be formed in the same grid pattern as conductive biasgrid layer 328. Color filter material 312 may be deposited in anysuitable pattern (e.g., a Bayer color filter pattern, acyan-yellow-green-magenta filter pattern, a red-green-blue-emeraldfilter pattern, or other suitable pattern). In some embodiments, colorfilter material 312 may be replaced with transparent filters formonochromatic image sensing. It should be noted that the dielectriccolor filter barriers may include electrically isolated metal portionsthat may reduce optical crosstalk caused by light rays that reach theimage plane at high incident angles.

Substrate 320 may include conductive substrate contacts 326 that maycontact conductive bias grid layer 328 and that may be formed in theback surface of lightly doped semiconductor region 322 using ionimplantation or diffusion. Conductive substrate contacts 326 may beformed directly across from ground contacts 308. Conductive substratecontacts 326 may be formed at each intersection point of conductive biasgrid layer 328, or may be formed at alternating intersection points ofconductive bias grid layer 328. Substrate 320 may further include activephotodiode regions 324 having a height D2 and being formed in lightlydoped semiconductor region 322. Image pixel array 300 may be formedusing a deep photodiode (DPD) process, and therefore height D2 of activephotodiode regions 324 may be greater than height D1 of activephotodiode regions 224 shown in FIG. 2. The thickness of substrate 320is therefore greater than the thickness of substrate 220 shown in FIG.2. This increase in thickness decreases the electric field magnitudebetween substrate contacts 326 and ground contacts 308.

Image pixel edge 330 may include a bond pad 332 that is connected toconductive bias grid layer 328, a bond pad 336 that is coupled toexternal voltage supply circuitry, and a wire bond 334 that is coupledbetween bond pad 332 and bond pad 336. External voltage supply circuitrymay apply a bias voltage to conductive bias grid layer 328 through wirebond 334 and bond pads 332 and 336. By applying a bias voltage toconductive bias grid layer 328, the electric field strength betweenconductive substrate contacts 326 and ground contacts 308 may beincreased. By increasing the electric field strength in this way, thereverse-bias strength of the p-n junction closest to the back-side ofthe semiconductor substrate may be increased. The negative effectscaused by increasing substrate thickness may thereby be nullified andelectrical isolation and charge collection efficiency may be preservedor improved.

FIG. 4 is a cross-sectional side view of an illustrative BSI image pixelarray showing conductive substrate contacts, a bias grid layer, metalcolor filter barriers, and a through-hole via edge interface inaccordance with an embodiment. As shown in FIG. 4, image pixel array 400may receive photons 402 and may include an image pixel array edge 430and a color filter 410 formed over a conductive bias grid layer 428 thatmay be formed on a back surface of substrate 420. Conductive bias gridlayer 428 may be formed from conductive material (e.g., aluminum,tungsten, copper, other suitable metals, etc.), conductive oxides (e.g.,indium tin oxide, bismuth titanate, etc.), or doped semiconductormaterial. An array of microlenses 404 may be formed over color filterarray 410. Ground contacts 408 and transfer gates 406 may be formed on afront surface of semiconductor 420. Ground contacts 408 may be formedfrom conductive material (e.g., aluminum, tungsten, copper, or othersuitable metals) and may be formed over a doped portion of substrate420.

Color filter array 410 may include color filter material 412 that isdeposited in cavities of color filter barrier grid 414. In theillustrative example of FIG. 4, color filter barrier grid 414 is formedfrom a conductive material such as metal (e.g., aluminum, tungsten,copper, etc.). Each cell in color filter barrier grid 414 may define oneor more individual pixels in image pixel array 400. Color filter barriergrid 414 may be formed directly over conductive bias grid layer 428 andmay be formed in the same grid pattern as conductive bias grid layer428. Color filter barrier grid 414 and conductive bias grid layer 428may be formed from the same material in the same process step. Colorfilter material 412 may be deposited in any suitable pattern.

Substrate 420 may include conductive substrate contacts 426 that maycontact conductive bias grid layer 428 and that may be formed in theback surface of lightly doped semiconductor region 422 using ionimplantation or diffusion. Conductive substrate contacts 426 may beformed directly across from ground contacts 408. Conductive substratecontacts 426 may be formed at each intersection point of conductive biasgrid layer 428, or may be formed at alternating intersection points ofconductive bias grid layer 428. Substrate 420 may further include activephotodiode regions 424 having a height D2 and being formed in lightlydoped semiconductor region 422. Image pixel array 400 may be formedusing a DPD process, and therefore height D2 of active photodioderegions 424 may be greater than height D1 of active photodiode regions224 shown in FIG. 2. The thickness of substrate 420 is therefore greaterthan the thickness of substrate 220 shown in FIG. 2. This increase inthickness decreases the electric field magnitude between substratecontacts 426 and ground contacts 408.

Image pixel edge 430 may include a through-hole via 434 that is coupledbetween conductive bias grid layer 428 and metal contact 436. Metalcontact 436 may be formed in an additional substrate 438 that may beattached to substrate 420. Metal contact 436 may be coupled to externalvoltage supply circuitry. External voltage supply circuitry may apply abias voltage to conductive bias grid layer 428 through via 434 and metalcontact 436. By applying a bias voltage to conductive bias grid layer428, the electric field strength between conductive substrate contacts426 and ground contacts 408 may be increased. By increasing the electricfield strength in this way, the reverse-bias strength of the p-njunction closest to the back-side of the semiconductor substrate may beincreased. The negative effects caused by increasing substrate thicknessmay thereby be nullified and electrical isolation and charge collectionefficiency may be preserved or improved.

FIG. 5 is a top-down view of an illustrative BSI image pixel arrayshowing contacts located at each intersection of a bias grid under aBayer color filter array in accordance with an embodiment. As shown inFIG. 5, image pixel array 500 may include pixels 502 that are arrangedin rows and columns and conductive substrate contacts 504 located ateach intersection of bias grid 506 and formed in the back surface of asemiconductor substrate. Bias grid 506 is formed over and electricallyconnected to substrate contacts 504. Each conductive substrate contact504 corresponds to a respective ground contact (e.g. ground contacts 308in FIG. 3) located on the opposite side of the semiconductor substrate.When a voltage is applied to bias grid 506, the electric field betweenconductive substrate contacts 504 and their respective ground contacts(e.g. ground contacts 308 in FIG. 3) increases in strength and improvesthe reverse-bias of back-side p-n junctions in pixels 502. In thisarrangement, each pixel may have an associated substrate contact,allowing for individual control of the reverse-bias of the backside p-njunctions in the pixel.

The example of FIG. 5 in which each pixel is provided with an associatedsubstrate contact 504 is merely illustrative. If desired, a substratecontact may be shared among multiple pixels, as shown in FIG. 6.

FIG. 6 is a top-down view of an illustrative BSI image pixel arrayshowing shared contacts located at alternating intersections of a biasgrid under a Bayer color filter array in accordance with an embodiment.As shown in FIG. 6, image pixel array 600 may include pixels 602 thatare arranged in rows and columns and conductive substrate contacts 604located at alternating intersections of bias grid 606 and formed in theback surface of a semiconductor substrate. Bias grid 606 is formed overand electrically connected to substrate contacts 604. Each conductivesubstrate contact 604 corresponds to a respective ground contact (e.g.ground contacts 308 in FIG. 3) located on the opposite side of thesemiconductor substrate. When a voltage is applied to bias grid 606, theelectric field between conductive substrate contacts 604 and theirrespective ground contacts (e.g. ground contacts 308 in FIG. 3)increases in strength and improves the reverse-bias of back-side p-njunctions in pixels 602. In this arrangement, multiple pixels share asingle substrate contact, which requires the formation of fewer totalsubstrate contacts.

FIG. 7 is a flow chart of illustrative steps involved in fabricating aBSI image pixel array having substrate contacts, a bias grid layer, anda grid of dielectric color filter barriers in accordance with anembodiment.

At step 702, conductive contacts (e.g., conductive substrate contacts326 in FIG. 3) are formed in the back side of a silicon substrate (e.g.,silicon substrate 320 in FIG. 3). It should be noted that this exampleis merely illustrative. Other embodiments may include substrates made ofsemiconductor materials other than silicon (e.g., silicon carbide,gallium nitride, gallium arsenide, etc.).

At step 704, a thin conductive layer (e.g., conductive bias grid layer328 in FIG. 3) is formed in a grid pattern over the contacts on the backside of the silicon substrate.

At step 706, a grid of dielectric color filter barriers (e.g.,dielectric color filter barriers 314 in FIG. 3) are formed over the thinconductive layer.

At step 708, color filter elements (e.g., color filter material 312 inFIG. 3) are deposited in cavities in the grid of color filter barriersto form a color filter array (e.g., color filter array 310 in FIG. 3).

At step 710, an array of microlenses (e.g., array of microlenses 304 inFIG. 3) are formed over the color filter array.

FIG. 8 is a flow chart of illustrative steps involved in fabricating aBSI image pixel array having substrate contacts and a bias grid of metalcolor filter barriers in accordance with an embodiment.

At step 802, conductive contacts (e.g., conductive substrate contacts426 in FIG. 4) are formed in the back side of a silicon substrate (e.g.,silicon substrate 420 in FIG. 4). It should be noted that this exampleis merely illustrative. Other embodiments may include substrates made ofsemiconductor materials other than silicon (e.g., silicon carbide,gallium nitride, gallium arsenide, etc.).

At step 804, a grid of metal color filter barriers (e.g., dielectriccolor filter barriers 414 in FIG. 4) are formed over the contacts on theback side of the silicon substrate.

At step 806, color filter elements (e.g., color filter material 412 inFIG. 4) are deposited in cavities in the grid of color filter barriersto form a color filter array (e.g., color filter array 410 in FIG. 4).

At step 808, an array of microlenses (e.g., array of microlenses 404 inFIG. 4) are formed over the color filter array.

FIG. 9 is a block diagram of a processor system employing at least someof the embodiments of the image pixel array in FIGS. 3-6. Device 984 maycomprise the elements of device 10 (FIG. 1) or any relevant subset ofthe elements. Processor system 900 is exemplary of a system havingdigital circuits that could include imaging device 984. Without beinglimiting, such a system could include a computer system, still or videocamera system, scanner, machine vision, vehicle navigation, video phone,surveillance system, auto focus system, star tracker system, motiondetection system, image stabilization system, and other systemsemploying an imaging device.

Processor system 900, which may be a digital still or video camerasystem, may include a lens or multiple lenses indicated by lens 996 forfocusing an image onto an image sensor, image sensor array, or multipleimage sensor arrays such as image sensor 16 (FIG. 1) when shutterrelease button 998 is pressed. Processor system 900 may include acentral processing unit such as central processing unit (CPU) 994. CPU994 may be a microprocessor that controls camera functions and one ormore image flow functions and communicates with one or more input/output(I/O) devices 986 over a bus such as bus 990. Imaging device 984 mayalso communicate with CPU 994 over bus 990. System 900 may includerandom access memory (RAM) 992 and removable memory 988. Removablememory 988 may include flash memory that communicates with CPU 994 overbus 990. Imaging device 984 may be combined with CPU 994, with orwithout memory storage, on a single integrated circuit or on a differentchip. Although bus 990 is illustrated as a single bus, it may be one ormore buses or bridges or other communication paths used to interconnectthe system components.

Various embodiments have been described illustrating image sensor havingan array of image sensor pixels that includes conductive contacts and aconductive grid layer that may receive a bias voltage to bolsterelectric field strength with a substrate in the image sensor, which mayhelp improve electrical isolation and charge collection efficiency.

An electronic device having an array of image sensor pixels may includephotosensitive elements formed in a semiconductor substrate having firstand second opposing surfaces, conductive contacts formed in the firstsurface of the semiconductor substrate, a conductive layer formed overthe conductive contacts, a color filter array formed on the firstsurface of the semiconductor substrate, an array of microlenses formedover the color filter array, and ground contacts formed on the secondsurface of the semiconductor substrate.

In an embodiment, the electronic device may include a bond pad formed atan edge of the array of image sensor pixels and a wire bond that iscoupled between the bond pad and external voltage supply circuitry thatis configured to apply a voltage to the conductive layer. In anotherembodiment, the electronic device may include a conductive via that isformed in the semiconductor substrate at an edge of the array of imagesensor pixels and that is coupled between the conductive layer andexternal voltage supply circuitry that is configured to apply a voltageto the conductive layer.

In an embodiment, the color filter array may include a grid ofdielectric material having an array of openings and color filterelements that are formed in the openings in the grid of dielectricmaterial.

In an embodiment, the conductive contacts may be formed belowintersections of the grid of dielectric material.

In an embodiment, the conductive layer may be formed in a grid patternthat overlaps the grid of dielectric material.

In an embodiment, the conductive layer may be formed from dopeddielectric material. In another embodiment, the thin conductive layermay be formed from metal.

An image sensor having an array of pixels may include photodiodes formedin a semiconductor substrate, conductive contacts formed in a first sideof the semiconductor substrate, a conductive grid layer formed over andin contact with the contacts on the first side of the semiconductorsubstrate that may include a grid of color filter barriers havingintersections and being formed over the conductive grid layer, amicrolens array formed over the color filter array, and metal contactsformed on a second side of the semiconductor substrate.

In an embodiment, the color filter array may include color filterelements that are deposited in cavities in the grid of color filterbarriers.

In an embodiment, the grid of color filter barriers conductive. Inanother embodiment, the grid of color filter barriers may be insulatingand may include electrically isolated metal.

In an embodiment, the conductive contacts may be formed under eachintersection of the grid of color filter barriers. In anotherembodiment, the conductive contacts may be formed under less than all ofthe intersections in the grid of color filter barriers.

In an embodiment, the color filter array may be a Bayer color filterarray.

In an embodiment, the conductive contacts may overlap the metalcontacts.

A system may include a central processing unit, memory, a lens,input-output circuitry, and an imaging device. The imaging device mayinclude an array of pixels arranged in rows and columns. The array ofpixels may include photodiodes formed in a substrate, conductivecontacts formed in a first side of the substrate, a color filter arrayformed on the first side of the substrate, an array of microlensesformed over the color filter array, and ground contacts formed on asecond side of the substrate. The color filter array may include a gridof metal barriers formed over the conductive contacts.

In an embodiment, the conductive contacts may be formed directlyopposite the ground contacts. In another embodiment, the grid of metalbarriers may be coupled to external voltage supply circuitry. Theexternal voltage supply circuitry may be configured to apply a voltageto the grid of metal barriers. In another embodiment, the conductivecontacts may be formed from doped semiconductor material.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention. Theforegoing embodiments may be implemented individually or in anycombination.

What is claimed is:
 1. An electronic device having an array of imagesensor pixels, comprising: photosensitive elements formed in asemiconductor substrate having first and second opposing surfaces;conductive contacts formed in the first surface of the semiconductorsubstrate; a conductive layer formed over the conductive contacts; acolor filter array formed on the first surface of the semiconductorsubstrate; an array of microlenses formed over the color filter array;and ground contacts formed on the second surface of the semiconductorsubstrate.
 2. The electronic device defined in claim 1, furthercomprising: a bond pad formed at an edge of the array of image sensorpixels; and a wire bond coupled between the bond pad and externalvoltage supply circuitry, wherein the external voltage supply circuitryis configured to apply a voltage to the conductive layer.
 3. Theelectronic device defined in claim 1, further comprising: a conductivevia that is formed in the semiconductor substrate at an edge of thearray of image sensor pixels and that is coupled between the conductivelayer and external voltage supply circuitry, wherein the externalvoltage supply circuitry is configured to apply a voltage to theconductive layer.
 4. The electronic device defined in claim 1, whereinthe color filter array further comprises: a grid of dielectric materialhaving an array of openings; and color filter elements formed in theopenings in the grid of dielectric material.
 5. The electronic devicedefined in claim 4, wherein the conductive contacts are formed belowintersections in the grid of dielectric material.
 6. The electronicdevice defined in claim 4, wherein the conductive layer is formed in agrid pattern that overlaps the grid of dielectric material.
 7. Theelectronic device defined in claim 1, wherein the conductive layercomprises doped dielectric material.
 8. The electronic device defined inclaim 1, wherein the conductive layer comprises metal.
 9. An imagesensor having an array of pixels, comprising: photodiodes formed in asemiconductor substrate; conductive contacts formed in a first side ofthe semiconductor substrate; a conductive grid layer formed over and incontact with the conductive contacts on the first side of thesemiconductor substrate; a color filter array formed on the first sideof the semiconductor substrate comprising a grid of color filterbarriers formed over the conductive grid layer; a microlens array formedover the color filter array; and metal contacts formed on a second sideof the semiconductor substrate.
 10. The image sensor defined in claim 9,wherein the color filter array further comprises: color filter elementsdisposed in cavities in the grid of color filter barriers.
 11. The imagesensor defined in claim 10, wherein the grid of color filter barrierscomprises a conductive grid of color filter barriers.
 12. The imagesensor defined in claim 10, wherein the grid of color filter barrierscomprises electrically isolated metal formed in an insulating grid ofcolor filter barriers.
 13. The image sensor defined in claim 10, whereinthe conductive contacts are formed under respective intersections in thegrid of color filter barriers.
 14. The image sensor defined in claim 10,wherein the grid of color filter barriers has a plurality ofintersections and wherein the conductive contacts are formed under lessthan all of the intersections in the grid of color filter barriers. 15.The image sensor defined in claim 9, wherein the color filter arraycomprises a Bayer color filter array.
 16. The image sensor defined inclaim 9, wherein the conductive contacts overlap the metal contacts. 17.A system, comprising: a central processing unit; memory; a lens;input-output circuitry; and an imaging device, wherein the imagingdevice comprises: an array of pixels arranged in rows and columns,wherein the array of pixels comprises: photodiodes formed in asubstrate; conductive contacts formed in a first side of the substrate;a color filter array formed on the first side of the substrate, whereinthe color filter array comprises a grid of metal barriers formed overthe conductive contacts; an array of microlenses formed over the colorfilter array; and ground contacts formed on a second side of thesubstrate.
 18. The system defined in claim 17, wherein the conductivecontacts are formed directly opposite the ground contacts.
 19. Thesystem defined in claim 17, wherein the grid of metal barriers iscoupled to external voltage supply circuitry that is configured to applya voltage to the grid of metal barriers.
 20. The system defined in claim17, wherein the conductive contacts comprise doped semiconductormaterial.